How do I alter the timezone of a DateTime in Ruby?

Using Rails 3, you're looking for DateTime.change()

dt = DateTime.now
=> Mon, 20 Dec 2010 18:59:43 +0100
dt = dt.change(:offset => "+0000")
=> Mon, 20 Dec 2010 18:59:43 +0000

d = DateTime.now
puts [ d, d.zone ]
#=> 2010-12-17T13:28:29-07:00
#=> -07:00

d2 = d.new_offset(3.0/24)
puts d2, d2.zone
#=> 2010-12-17T23:28:29+03:00
#=> +03:00

Edit: This answer doesn't account for the information provided by comment to another answer that the desire is to have the reported 'hours' be the same after the time zone change.

As @Sam pointed out, changing offset is not sufficient and would lead to errors. In order to be resistant to DST clock advancements, the conversion should be done in a following way:

d = datetime_to_alter_time_zone
time_zone = 'Alaska'

DateTime.new
  .in_time_zone(time_zone)
  .change(year: d.year, month: d.month, day: d.day, hour: d.hour, min: d.min, sec: d.sec)

Using a number offset e.g. '+1000' wont work all year round because of daylight savings. Checkout ActiveSupport::TimeZone. More info here